A 44-year-old woman with a two-week history of a nonproductive cough that worsens at night, with no fever or weight loss, began after a sore throat and catarrhal phase. The most likely diagnosis and the next best test?

• A. Bronchitis; chest X-ray
• B. Pertussis; Bordetella pertussis PCR
• C. Common cold; no testing needed
• D. Pneumonia; sputum culture

Answer: (B)

Prolonged, predominantly nonproductive cough that worsens at night after an upper respiratory–type illness with a brief catarrhal phase points toward pertussis. In adults, the classic loud whoop isn’t always present, but the paroxysmal coughing spells with possible posttussive vomiting are characteristic and highly infectious, making timely diagnosis important.

The best test to confirm pertussis early in the course is Bordetella pertussis PCR on a nasopharyngeal specimen. PCR is highly sensitive in the early phase and yields rapid results, helping to confirm infection quickly. Culturing the organism is possible but slower and less sensitive in adults, and serology is less helpful in the acute setting. Chest imaging would often be normal or show non-specific findings in pertussis, which doesn’t distinguish it from bronchitis or the common cold; pneumonia would typically present with fever and focal findings, which aren’t described here.

So, the most likely diagnosis is pertussis, and the next best test is Bordetella pertussis PCR.